The invention relates to a test device for electrical testing of a unit under test, in particular for the testing of wafers. The device has a contact head which can be associated with the unit under test and is provided with contact elements which are in the form of pins and form a contact pin arrangement. The device further has an electrical connecting apparatus, which has contact surfaces which make a touching contact with those ends of the contact elements which face away from the test plane accommodating the unit under test. The device also has a supporting apparatus which is arranged on the side of the connecting apparatus facing away from the contact head.
Electrical test devices of this type initially are used to make electrical contact with a unit under test in order to test its functionality and serviceability. The electrical test device produces electrical connections to the unit under test, that is to say it makes contact on the one hand with electrical connections on the unit under test and on the other hand provides electrical contacts which are connected to a test system which supplies electrical signals to the unit under test via the test device in order to carry out, for example, resistance measurements, current and voltage measurements and so on for functional testing. Since the electrical units under test are often very small electronic components, for example electronic circuits on a wafer from which electronic components are manufactured, the contact elements, which are in the form of pins, on the test head have extremely small dimensions. In order now to provide a connection capability to the test system that has been mentioned, the contact elements of the test head make touching contact with a connecting apparatus which provides conversion to a greater contact separation and to this extent, allows the connection of electrical connecting cables which lead to the test system. During testing, the unit under test is located on the test plane, and the test device is lowered axially, preferably in the vertical direction, onto the unit under test. The first ends of the contact elements, which are in the form of pins, make contact with the unit under test. The other ends of the contact elements, which are in the form of pins, meet the contact surfaces of the connecting apparatus. The supporting apparatus applies the contact force, in the axial direction, which is applied to the unit under test in order to make the contacts that have been mentioned. At the same time, the support for the connecting apparatus, which is often flat, on the supporting apparatus counteracts any bending of the connecting apparatus resulting from reaction forces caused by making contact with the unit under test. Nevertheless, manufacturing tolerances and external conditions mean that there is no guarantee that the contact pins of the contact pin arrangement will make contact with the unit under test with approximately the same contact pressure. Even a minor deviation from the planar condition of a component can lead to a certain proportion of the contact pins making contact with the unit under test at an early stage during the lowering process, and also being pressed against the unit under test with a greater contact pressure than the other contact pins during testing. In the worst case, it is possible in the case of the known test device for certain contact pins to make no contact with the unit under test, even though other contact pins are resting against the unit under test with the desired contact pressure. The contact pins compensate for minor discrepancies themselves, since they are preferably in the form of bent wires on which forces act in the longitudinal direction while making contact and which are in consequence easily elastically bent in the form of an arc. However, this compensation is not always adequate.